In order to reduce greenhouse gas to alleviate the issues of global warming and to prepare for the diminishing supply of fossil fuels, the use of renewable energy resources from waste materials is increasingly critical to the health of the earth since waste materials can be converted into renewable energy as well as hydrocarbon compounds for the chemical industry. Carbonaceous materials, such as biomass and solid wastes and the like, can be thermochemically converted to materials that can replace fossil fuels. These thermochemical processes are generally classified as combustion, pyrolysis and gasification.
During the last several decades, pyrolysis and gasification have been extensively researched to convert low value and highly distributed solid biomass and/or wastes into different products. These products include a bio-liquid known as bio-oil, a solid residue known as biochar, and a gaseous mixture known as synthetic gas. Synthetic gas or syngas mainly includes hydrogen (H2), carbon monoxide (CO), methane (CH4), carbon dioxide (CO2) and relatively lower molecular weight of hydrocarbon compounds (tar). Furthermore, these products can be utilized as industrial feedstocks i.e. for combined heat and power (CHP) production, liquid fuels synthesis, chemical industry, hydrogen production, soils amelioration and carbon sequestration.
Gasification process provides a major pathway to convert biomass and/or waste into synthetic gas. The gasifying agents can be air, O2, steam, H2, CO2, inert gases or mixtures thereof. Air, being a cheap and widely used gasifying agent, contains high amount of nitrogen (about 79 wt % in Air). However, air has the diluting effect and thus lowers the heating value of the produced syngas. If Oxygen is used as gasifying agent, the heating value of syngas will increase. However, the total costs will also increase because of the requirement of utilizing air separation unit to produce pure Oxygen. Partial combustion of biomass and/or waste with air or O2 could supplies the required heat energy for drying as well as the endothermic gasification reactions [1 Basu P. Combustion and gasification in fluidized. Boca Raton, Fla.: CRC Press; 2006. p. 59-101].
If steam is used as the gasifying agent, the heating value and hydrogen content of syngas can be increased to about 10-15 MJNm−3 [2, Rapagna S, Jand N, Kiennemann A, Foscolo P U. Steam gasification of biomass in a fluidised-bed of olivine particles. Biomass & Bioenergy 2000]. However, indirect or external heat supply is required to drive the endothermic thermochemical process, and the relatively high content of tar in the synthetic gas requires additional tar reforming/removing steps to avoid plugging-up in downstream processing equipment.
Conventionally, there are three main types of gasifiers: fixed bed, moving bed and fluidized bed gasifiers. Both fixed bed and moving bed gasifiers produce synthetic gas which normally entrained with substantial quantities of tar and/or char fines and/or particulates due to the low and non-uniform heat and mass transfer between solid biomass/waste and gasifying agent. In effect, this limits the processing capacity of the fixed bed and moving bed gasifiers to small scale. In operation, the tar condensation, entrained particulates and char fines in the condensed liquid, material agglomeration, and ash-related problems present major challenges to the operation of fixed bed and moving bed gasifiers as well as the utilization of end products. Therefore, effective solutions are needed to achieve safe and effective operations for these types of gasification. Fluidized bed gasifiers, which consist of hot inert materials such as sand, dolomite, olivine and the like, have been used widely as heat transfer agents in biomass fluidized bed gasification. Fluidized bed gasification process can achieve a high heating rate, uniform heating, and high processing capacity, thus enabling the utilization of fluidized bed gasification systems in medium and large scale plants. In addition, the relatively lower operating temperature of fluidized bed gasifiers in comparison to that of fixed bed gasifiers also help reducing the ash-related issues. However, due to the fluidization of biomass and heat transfer agents, fine particles resulted from attrition can be entrained in the produced syngas which required effective filtering in downstream process. [3 L. Wang et al. Contemporary issues in thermal gasification of biomass and its application to electricity and fuel production-BIOMASS AND BIOENERGY 32 (2008) 573-581]. Furthermore, the fluidizing condition imposes upper limits to the size of feedstock materials, normally in the range of less than about 3 mm. Consequently, pretreatment of feedstock materials such as pre-drying and size reduction are generally required. Depending on the nature of incoming feedstocks, the pretreatment process could be excessive, thus increasing the operational complexity and cost.
Tar has been identified as a major issue to the operation of thermochemical systems as well as the utilization of end products. During gasification, part of the biomass and/or waste is converted to char and tar instead of syngas. Effective utilization of syngas as a fuel for internal combustion engines, gas turbines and fuel cells for heat and power generation and as a feedstock for the synthesis of liquid fuels and chemicals depends highly on downstream gas conditioning technologies of the produced syngas.
Operating parameters such as gasification temperature, pressure and the equivalence ratio (ER, the ratio of O2 required for gasification to O2 required for stoichiometric combustion of a given amount of biomass) also have an effect on char and tar formation. High gasification temperature can achieve a high carbon conversion of the biomass and low tar content in syngas. However, high operating temperatures decrease the energy efficiency and increase the risk of ash sintering and agglomeration. Hasler and Nussbaumer [4] [Hasler P, Nussbaumer T. Gas cleaning for IC engine applications from fixed bed biomass gasification. Biomass & Bioenergy 1999; 16:385-95] observed that a 90% particle removal was easier to achieve than a 90% tar removal using the mechanical methods. Therefore, tar elimination is a key challenge for a successful application of biomass-derived syngas. Although nickel and other stable metal catalysts can almost completely remove tar, however, they are expensive, easily deactivated by coke formation, poisoned by H2S and sintered by ash melting at high temperature. Both alkali and dolomite catalysts are cheaper but they cannot remove all tar components in the syngas. Swierczynski et al. [5] developed a combined Ni-dolomite catalyst for steam reforming of tar using metallic nickel as an active phase grafted on dolomite. Their results showed that 97% of tar removal was obtained at a reforming temperature of 750° C. and a space velocity of 12,000 h−1 and no obvious deactivation of catalyst were observed in 60 h tests.
Gasification provides a competitive way to convert diverse, highly distributed and low-value biomass and/or wastes to syngas for the combination of heat and power generation, synthesis of liquid fuels, and production of hydrogen (H2). Co-firing of syngas in existing pulverized coal and natural gas combustors has been successfully commercialized. Fluidized bed gasification with steam and indirect or external heat supply demonstrated a promising way to improve the syngas yield and quality. Catalysts are widely used for syngas cleaning and for production of liquid fuels and H2 from syngas. Nevertheless, improvement is still needed to improve syngas quality for its commercial uses in a high energy efficient heat and power generator i.e. gas turbines or fuel cells, and the production of liquid fuels, chemicals and H2.
With respect to incoming feedstocks, the moisture content, the hydrogen content deficiency and the high level Oxygen content are other challenges in a typical thermochemical conversion process of biomass and/or waste. The evaporation of the moisture from the incoming feedstocks is generally considered as an undesired energy penalty and also reduces the conversion efficiency of the process. Furthermore, besides the moisture content issues, biomass and solid waste materials are generally deficient of hydrogen and contained high level of oxygen. These are among the main reasons for the undesirable characteristics of the produced bio-oil from biomass.
Pyrolysis is another major pathway for converting biomass and/or waste into fuels and/or chemicals in a thermochemical platform. One of the main products from a conventional fast pyrolysis process is bio-oil. Bio-oil generally is a dark brown liquid with properties that are acidic, immiscible with fossil fuels. It has relatively high oxygen content and water content in comparison to fossil fuels. Furthermore, as bio-oil ages, it becomes unstable due to polymerization and phase separation during storage. These are characteristics that cause bio-oil to be generally not compatible with the existing refinery equipment or processes which are conventionally used for processing crude oil to transportation fuels. In short, the major challenges in utilization of bio-oil are the instability of the highly reactive bio-oils during storage which limits the applications of bio-oils as biofuels; the high oxygen content in bio-oil, presents as oxygenated compounds, requires a sufficient amount of hydrogen for upgrading the bio-oil into transportation fuels via hydroprocessing which makes the process more expensive due to the costs of hydrogen production; the relative high water content and the immiscibility of bio-oil with petroleum crude make co-refining of bio-oil with petroleum crude difficult and the acidity of bio-oil presents corrosion issues to existing fuel-infrastructure and engines.
Therefore, bio-oils with improved properties, such as lower oxygen content, lower water content and less acidic, are highly desirable. The application of bio-oil as a replacement for traditional chemicals is always a challenge due to its complex composition. Bio-oil is a liquid consisting of several hundred chemical components. Most of the components are presented in low concentration. Therefore separation or fractionation of bio-oil is a promising approach to convert biomass and/or wastes to liquid fuels and/or chemical. At present, the use of biomass and/or waste resources to produce fungible fuels is continuously advancing.
To date, several research efforts to improve the properties of bio-oil have been focused toward post-pyrolysis treatment. This treatment upgrades the liquid bio-oils obtained from pyrolysis includes hydroprocessing and/or hydrotreating, catalytic cracking, thermal cracking and the like. On the other hand, less effort has been focused on in situ and/or integrated upgrading of pyrolysis vapor before it is condensed into liquid. This lack of progress in the integrated upgrading of pyrolysis vapor is mainly due to: (1) the complex and heterogeneous nature of the feedstock materials, (2) the complex thermochemical processes involved, and (3) the complexity of the properties of end products.
Conventionally, vapor phase upgraded bio-oil can be combined with hydrogen at 255°-410° C. and at about 2,000 psig pressure to convert the upgraded bio-oil to hydrocarbons, water, and gases over a fixed bed reactor. Depending upon the reactivity of the vapor phase upgraded bio-oil, two beds may be needed. The first bed is operated at the lower end of the temperature range to further reduce any remaining highly reactive compounds. The second bed is operated at the higher end of the temperature range, and possibly at a lower space velocity to allow complete deoxygenation.
The ideal goal is that the vapor phase upgraded bio-oil is of high enough quality so that only a single hydrotreater is needed. After cooling, the products are separated and the hydrocarbon product is distilled into hydrocarbon gases (C4−), gasoline range, diesel range, and heavy oil ranges materials. (Zacher et al. 2011).
ZSM-5 has been extensively studied for the upgrading of biomass pyrolysis vapors and its selectivity towards hydrocarbons is very well known. ZSM-5 had a more balanced performance with good selectivity towards hydrocarbons and an organics fraction yield on biomass. The oxygen was removed from the pyrolysis vapors in the form of CO2, CO and H2O which resulted in a subsequent reduction of the total liquid and organic fraction yields due to the transfer of carbon in the gas products, the formation of water and the formation of coke deposits on the catalyst surface. Among zeolites, ZSM-5 has been extensively investigated as a catalyst for biomass pyrolysis and found to dramatically change the composition of the bio-oils by both reducing the amounts of oxygenated compounds via deoxygenation reactions and simultaneously increasing the aromatic species, producing an organic fraction (bio-oil) that can be upgraded to gasoline and diesel type fuel. In addition, the molecular weight of the bio-oil is decreased. The use of ZSM-5 catalyst is reported to reduce oxygen content in bio-oil from 33 to 13%. Oxygen removal was found to take place as H2O at lower temperatures and as CO and CO2 at higher temperatures. The latter case is preferable, as more hydrogen would be accessible for hydrocarbon formation and consequently less carbon would deposit on the zeolite, while at the same time the water content of bio-oil is reduced. (Andrew J. Foster et. al, Optimizing the aromatic yield and distribution from catalytic fast pyrolysis of biomass over ZSM-5).
Referring to FIG. 1, a conventional fixed bed or moving bed gasifiers 100 is illustrated. There are multiple aspects of the operation which need to be considered. The major aspects are the moisture content, the physical dimension as well as the composition of the feedstock materials, the flows of the volatilized vapor relative to the flow of the feedstock materials i.e. updraft, downdraft or cross draft, the design and construction of the gasifiers, the operational parameters of the gasifier such as the type of processing agents i.e. air, oxygen, steam or other gases, the equivalent ratio which represents the ratio of the supplied oxygen to the stoichiometric oxygen for complete combustion in the case of air and/or oxygen is used as processing agent, the removal of the char, particulates and ash from the gasifier as well as from the produced synthetic gas (or syngas), the conditioning and utilization of the produced syngas.
In general, the complexity of the composition of the feedstock materials, the sequential nature of the devolatilization process with respect to the types of feedstock materials i.e. drying, devolatilization or pyrolysis, reduction and combustion, the complex evolution and reactive interaction of the volatilized vapor as well as the intrinsic mass and heat transfer limitation between feedstock materials and process gases have imposed several restrictions toward the desired operation of conventional gasifiers. As the results, the goal of stable operational conditions and the difficulties of increasing throughput capacity are the major challenges to the effort of commercializing large scale gasification plants.
Referring to FIG. 2, a catalytic hydropyrolysis system 200 taught by Terry L. Marker et. al (“Marker system”), US patent 2010/0256428 A1, is illustrated. Marker system 200 taught an approach which integrates the catalytic hydropyrolysis with hydroconversion and hydrocracking catalyst to produce fungible fuel from biomass. This approach also includes a specific way to produce hydrogen via steam reforming a portion of syngas which produced during the pyrolysis process combined with the pressure swing adsorption while maintaining a balance of the levels of decarboxylation, decarbonylation and hydrodeoxygenation to sustain the desired balanced process. This approach also requires the pretreatment of the incoming biomass materials to achieve the specific size to less than 3 mm to accommodate the fluidization process in addition to the consideration for the selection of fluidized bed materials i.e. Glass-ceramic sulfided NiMo, Ni/NiO, or Co-based catalysts in order to minimize the attrition effect. There are multiple configurations presented in the patent to teach the different combinations of the integration of the catalytic hydropyrolysis with hydroconversion and hydrocracking catalyst. In essence, each configuration provides a fixed process flow when the approach of Marker system 200 is implemented for the desired composition of the products.
Referring to FIG. 3, a phase and energy diagram 300 of water is illustrated. In thermochemical processes of carbonaceous materials, it has been known in the art that the moisture content of the feedstock materials is one of the important parameter with respect to the energy efficiency as well as the operational aspects of thermochemical processes. The moisture is often considered as an energy penalty due to the high energy required in vaporizing the moisture content of the feedstock materials. In practice, system designers often try to utilize the waste heat for the drying process to reduce the energy cost. In the total energy required for the drying process, the latent heat of vaporization generally requires a major portion of the total energy required.
Furthermore, when the vapor condenses, the latent heat will be released, thus, recovering the latent heat of vaporization could be an effective mean to minimize the energy penalty from the moisture content.
As a result of the above-mentioned issues and challenges, it is highly desirable to provide a thermochemical platform which is energy efficient, high degree of flexibility and high level of safety in operation, and economically feasible for the conversion of biomass and/or waste materials into high value and compatible end products.